1. Field of the Invention
The present invention relates to polymeric composite materials having improved surface properties. More particularly, the invention relates to a polymeric composite material having a decreasing concentration gradient of polymer matrix from the material's surface to the material's interior. The invention also includes a process for extracting polymer matrix from the interior of a composite material in order to increase the matrix concentration at the material's surface.
2. Description of the Prior Art
It is known that high modulus fibers, such as poly(p-phenylene terephthalamide) aramid fibers, may be incorporated into polymer matrix materials to form composites. For example, United Kingdom Patent Application, 2,195,672 A, published Apr. 13, 1988 discloses a process for fabricating composites comprising forming a network of microfibrils of a rigid polymer, a polymer which has the ability to form a liquid crystalline phase either in a solution or melt, and interpenetrating the microfibrils with a matrix material, such as a thermoplastic polymer, to form a composite.
Gabriel et al., coassigned, copending U.S. Pat. application Ser. No. 07/337,504, the disclosure of which is hereby incorporated by reference, discloses a process for making oriented, fibers of lyotropic polysaccharide/thermally-consolidatable polymer blends by spinning a biphasic solution containing at least about 55% and less than about 80% by weight of a lyotropic polysaccharide polymer, and at least about 20% and less than about 45% by weight of a thermally-consolidatable polymer. These fibers are particularly useful for composite applications.
Yang, coassigned, copending U.S. Pat. No. 5,011,643, the disclosure of which is hereby incorporated by reference, discloses a process for making oriented, fibers of para-aramid/thermally-consolidatable polymer blends by spinning a biphasic solution containing at least about 55% and less than about 80% by weight of a para-aramid polymer, and at least about 20% and less than about 45% by weight of a thermally-consolidatable polymer. These fibers are particularly useful for composite applications.
Uy, U.S. Pat. No. 4,810,735, the disclosure of which is hereby incorporated by reference, discloses fibers prepared from spinnable dopes comprising a first polymer selected from poly(paraphenylene benzobisthiazole) (PBT); poly(paraphenylene benzobisoxazole) (PBO); or poly-2,5-benzoxazole (AB-PBO) polymers, and a second polymer selected from a thermoplastic polymer or intractable polymer in a combined solvent of poly(phosphoric acid) and methanesulfonic acid or chlorosulfonic acid. These fibers are particularly useful for composite applications.
However, it was found that composite fibers, particularly those disclosed in the foregoing Gabriel et al., Yang, and Uy references, often have a deficient concentration of polymer matrix on their surfaces which causes poor adhesiveness, particularly in a cross-ply composite construction. It is often desirable to have a high concentration of polymer matrix at the surface of a polymeric composite material in order to improve adhesiveness, and other properties such as the surface finish. An objective of the present invention is to provide such materials having a decreasing concentration of polymer matrix from the material's surface to the material's interior.
Another objective of the invention is to provide a process for extracting polymer matrix from the interior of a polymeric composite material in order to increase the matrix concentration at the material's surface.